0:00:04 > 0:00:09Parasites. To most people they're simply revolting.
0:00:10 > 0:00:13Gut worms, came out of somebody.
0:00:13 > 0:00:15- About five or six different worms. - ALL: Eugh!
0:00:15 > 0:00:18Even the word "parasite" is an insult.
0:00:18 > 0:00:20How disgusting do you find that on a scale of one to ten?
0:00:22 > 0:00:27Disgusting they may be, but I think they're also truly extraordinary.
0:00:29 > 0:00:33They have astonishing life cycles
0:00:33 > 0:00:35and they are some of the most
0:00:35 > 0:00:37successful creatures on the planet.
0:00:39 > 0:00:43To find out more about these amazing animals, I'm going
0:00:43 > 0:00:47to deliberately infest myself with a range of parasites.
0:00:47 > 0:00:49It's like jumping off a cliff. Here we go.
0:00:51 > 0:00:52From the largest...
0:00:53 > 0:00:55..to the most deadly.
0:00:57 > 0:01:01By becoming a host I'm hoping to discover how the human body,
0:01:01 > 0:01:04this body, responds to parasite infection.
0:01:06 > 0:01:09I'm going to use my infestations to study first-hand
0:01:09 > 0:01:12our complex relationship with parasites.
0:01:12 > 0:01:14Ahh!
0:01:15 > 0:01:18Experience the costs
0:01:18 > 0:01:21and potential benefits of being infested.
0:01:23 > 0:01:28And even find out what they can tell us about our own origins.
0:01:45 > 0:01:49My first, of many, encounters with parasites starts here,
0:01:49 > 0:01:50in Nairobi, Kenya.
0:01:54 > 0:01:58I'm in search of an extraordinary human parasite.
0:01:58 > 0:01:59One which has been
0:01:59 > 0:02:02all but eliminated in the developed world.
0:02:02 > 0:02:06Now, most tourists come to Kenya in search of wildlife.
0:02:06 > 0:02:09Just down the road from here you can go to a safari park where you
0:02:09 > 0:02:11will see rhino and giraffes.
0:02:13 > 0:02:16I, however, have come in search of something smaller
0:02:16 > 0:02:18and much less attractive.
0:02:18 > 0:02:19Tapeworm.
0:02:21 > 0:02:25The tapeworm is one of nature's most successful parasites.
0:02:27 > 0:02:31There are thousands of different species of tapeworm each adapted
0:02:31 > 0:02:33to a particular host.
0:02:34 > 0:02:35RAT SQUEAKS
0:02:39 > 0:02:42The one I'm looking for is taenia saginata.
0:02:42 > 0:02:43CATTLE MOO
0:02:43 > 0:02:44The beef tapeworm.
0:02:46 > 0:02:49It's evolved to live in our guts.
0:02:49 > 0:02:53But its peculiar life cycle also involves cattle.
0:02:55 > 0:02:57So imagine you're in rural Kenya
0:02:57 > 0:03:00and you've got a great big tapeworm inside you.
0:03:00 > 0:03:02This tapeworm is producing eggs.
0:03:02 > 0:03:07One day you go down to a field and you excrete there -
0:03:07 > 0:03:09and you contaminate the grass.
0:03:09 > 0:03:13Then along comes a poor, unfortunate cow who eats that grass.
0:03:16 > 0:03:20Along with grass the cow eats the tapeworm eggs
0:03:20 > 0:03:22which hatch into larvae -
0:03:22 > 0:03:24and then enter the cow's bloodstream.
0:03:25 > 0:03:29It spreads through the blood to the muscle, perhaps the shoulders
0:03:29 > 0:03:33or the tongue and there it forms a really tough cyst.
0:03:33 > 0:03:37And it hangs around just waiting for the next stage, which is
0:03:37 > 0:03:40for the cow to be eaten by a human.
0:03:41 > 0:03:43COW MOOS
0:03:44 > 0:03:48The beef tapeworm can only grow to adulthood inside a human
0:03:48 > 0:03:52and for that reason it's hard to study.
0:03:52 > 0:03:55So I've decided to infest myself,
0:03:55 > 0:04:00as my contribution to research into these shy, retiring creatures.
0:04:00 > 0:04:02COW MOOS
0:04:02 > 0:04:04So, I need to find infected beef.
0:04:08 > 0:04:11But in Britain it's extremely unlikely you'll find a contaminated cow,
0:04:11 > 0:04:13which is why I've come to Kenya.
0:04:18 > 0:04:22But even here tapeworm is getting harder to find, as hygiene improves.
0:04:25 > 0:04:28To help me, I've teamed up with parasitology researcher,
0:04:28 > 0:04:29Judy Mwangi.
0:04:31 > 0:04:36Kind of felt like a good idea when we were planning this,
0:04:36 > 0:04:39but now we're actually doing it it seems quite strange.
0:04:39 > 0:04:43- Have you ever done anything like this before?- Oh, no, I haven't.
0:04:43 > 0:04:44THEY LAUGH
0:04:47 > 0:04:51We've had a tip-off that a meat inspector has found some infected beef.
0:04:53 > 0:04:58But to collect it we need to drive into one of Nairobi's tougher neighbourhoods.
0:05:01 > 0:05:05It looks quite a rough area out there.
0:05:05 > 0:05:08Oh, so what happens now, do we go to the slaughterhouse?
0:05:08 > 0:05:10No, we cannot walk to the slaughterhouse,
0:05:10 > 0:05:13we have to wait for him to come and bring us the sample.
0:05:13 > 0:05:16- OK.- Yes.- So he's definitely got a sample, has he?
0:05:16 > 0:05:18- He has now.- OK, all right.- We've been lucky.
0:05:18 > 0:05:20- Lucky! - Really lucky.
0:05:20 > 0:05:22I was kind of hoping you were going to say,
0:05:22 > 0:05:25- "No, tragically, they haven't found anything."- Hmm.
0:05:25 > 0:05:28- And will they let me into the slaughterhouse?- No, they will not.
0:05:28 > 0:05:30They're really sensitive about attitudes, anything...
0:05:30 > 0:05:33OK, and what about filming, presumably they won't?
0:05:33 > 0:05:34That is a definite no-no.
0:05:42 > 0:05:45Now we've got infected meat.
0:05:45 > 0:05:46We take it to the lab.
0:05:46 > 0:05:50So Judy can find the tapeworm cysts.
0:05:50 > 0:05:53- So, this is a cyst? - This is a cyst, yes.
0:05:53 > 0:05:55It's not at all what I was expecting.
0:05:55 > 0:05:57I was expecting something quite hard.
0:05:57 > 0:05:59- Hmm. - This is quite jelly-like isn't it?
0:05:59 > 0:06:02It is jelly-like, because if it's hard it's calcified
0:06:02 > 0:06:03so it's not viable.
0:06:04 > 0:06:07'I'm not particularly keen on eating raw meat.
0:06:07 > 0:06:11'So Judy is cutting away as much tissue as possible,
0:06:11 > 0:06:14'just leaving the tapeworm cyst itself.'
0:06:16 > 0:06:19It kind of looks like a partly fried egg, doesn't it?
0:06:19 > 0:06:21It does, sunny side up.
0:06:24 > 0:06:27Inside this cyst is a tiny living tapeworm larva.
0:06:28 > 0:06:32It's what I've come all the way to Kenya for
0:06:32 > 0:06:35and Judy has found three of them.
0:06:35 > 0:06:37Is that the right sort of number?
0:06:37 > 0:06:39I think so, one looks a bit dodgy.
0:06:39 > 0:06:42So one's a bit dodgy, two of them you think are OK?
0:06:42 > 0:06:44- Yes.- And that kind of just increases the risk...
0:06:44 > 0:06:45- ..one of them will take? - Yes, indeed.
0:06:45 > 0:06:47- What happens if two of them take? - The better.
0:06:47 > 0:06:50- SHE CHUCKLES - Better, better for whom?
0:06:50 > 0:06:52- Can I...offer you one?- No, you're all right.
0:06:55 > 0:07:00Back at the hotel, the time has come for me to give myself tapeworm.
0:07:02 > 0:07:06I'm feeling mildly apprehensive.
0:07:06 > 0:07:09I think I'm confident that it should be safe.
0:07:09 > 0:07:11It is a very strange thought that there are millions of people
0:07:11 > 0:07:14who are infected by parasites who don't want to be
0:07:14 > 0:07:16and I'm about to deliberately infect myself.
0:07:16 > 0:07:20But I hope it's not something I'm going to regret.
0:07:20 > 0:07:22Cheers.
0:07:24 > 0:07:27It's like jumping off a cliff. Here we go.
0:07:29 > 0:07:31Ah!
0:07:32 > 0:07:34There they go.
0:07:37 > 0:07:41In just a few seconds, the tapeworm cysts are in my stomach.
0:07:41 > 0:07:45That's full of acids that normally help to protect us from disease.
0:07:48 > 0:07:52But the tapeworm uses our defences to its advantage.
0:07:53 > 0:07:58The acids dissolve the outer case of the cyst, releasing the worm inside.
0:07:59 > 0:08:02Which then passes into my intestines looking for a home.
0:08:04 > 0:08:07My time in Kenya is over now!
0:08:07 > 0:08:10I've done what I needed to do. Now to go home.
0:08:21 > 0:08:27If they survive, each cyst will release a tapeworm scolex.
0:08:27 > 0:08:30Equipped with four suckers which it will use to latch on to my guts.
0:08:33 > 0:08:36Then it starts to grow.
0:08:36 > 0:08:39New segments emerging from the scolex.
0:08:43 > 0:08:46As it grows the segments will get bigger and more mature.
0:08:48 > 0:08:50Whilst new segments appear near the head.
0:08:53 > 0:08:55And I'll then use a miniature pill camera
0:08:55 > 0:08:58to see what's actually growing inside me.
0:09:02 > 0:09:06All this is only possible inside the human body.
0:09:06 > 0:09:10This particular tapeworm is so perfectly adapted to the exact
0:09:10 > 0:09:14conditions of the human gut that it can't grow anywhere else.
0:09:18 > 0:09:22That degree of specialisation may seem bizarre
0:09:22 > 0:09:24but it's actually common in parasites.
0:09:26 > 0:09:29There's a type of flatworm that can only
0:09:29 > 0:09:32live inside the lens of a stickleback's eye.
0:09:34 > 0:09:38And a mite that only lives inside a moth's ear.
0:09:45 > 0:09:49And perhaps the strangest example of extreme specialisation
0:09:49 > 0:09:53turned up here at the Horniman Museum in South London.
0:09:55 > 0:09:57One morning, someone brought them something
0:09:57 > 0:09:59they'd found in their fish supper.
0:10:01 > 0:10:05They basically brought us someone's evening meal on a plate.
0:10:05 > 0:10:09So it was a...a cooked fish that the person had started to eat.
0:10:11 > 0:10:13Halfway through their meal,
0:10:13 > 0:10:16they discovered this inside the fish's mouth.
0:10:16 > 0:10:20It was enough to put anyone off their food.
0:10:20 > 0:10:24So this is cymothoa exigua, or the fish tongue louse,
0:10:24 > 0:10:27and it has a really unusual parasitic lifestyle.
0:10:27 > 0:10:30It enters the gills of the fish
0:10:30 > 0:10:33and then attaches to the tongue-like structure of the fish
0:10:33 > 0:10:36and kind of clings on and pierces the tongue structure
0:10:36 > 0:10:38with its big claws
0:10:38 > 0:10:41and then basically feeds off the blood of the tongue,
0:10:41 > 0:10:44which eventually just becomes quite shrivelled
0:10:44 > 0:10:48and ends up being just a little stump that the fish tongue louse
0:10:48 > 0:10:51then moves further into the mouth, and basically
0:10:51 > 0:10:54starts to live as if it was the tongue of the fish.
0:10:54 > 0:10:58So, manipulating food around the mouth.
0:10:58 > 0:11:02The fish tongue louse is a particularly extreme example
0:11:02 > 0:11:07of a parasite that's adapted its body to fit its host's anatomy.
0:11:09 > 0:11:14But we also have parasites that have adapted to fit in with us.
0:11:14 > 0:11:16Including...lice.
0:11:18 > 0:11:21We have no fewer than three species of lice.
0:11:21 > 0:11:25And the story of how these three species evolved can tell us
0:11:25 > 0:11:28something remarkable about our own evolution.
0:11:33 > 0:11:37The most familiar human louse is the head louse.
0:11:41 > 0:11:47To see its unique characteristics I need my second infestation
0:11:47 > 0:11:50so I'm off to a de-lousing salon in North London.
0:11:51 > 0:11:54- So to remove the head lice... we use a normal Hoover...- Yep.
0:11:54 > 0:11:57..with a specialist attachment, so it's got
0:11:57 > 0:12:00a comb on the end of it there and then we collect the lice in there.
0:12:00 > 0:12:03Great, OK, well, that's good because I actually want live...
0:12:03 > 0:12:04OK!
0:12:04 > 0:12:07So, I'm hoping there's going to be a few to... Yeah...
0:12:07 > 0:12:09- Well, I'm sure there will be. - Oh, we shall see, OK.
0:12:09 > 0:12:11HOOVER STARTS
0:12:11 > 0:12:14Are you at all worried they're going to jump onto you?
0:12:14 > 0:12:17They don't jump and they don't fly. The only way they move is by crawling, so...
0:12:17 > 0:12:21- Yeah, it makes it... - If I put my head next to hers they could crawl across.
0:12:21 > 0:12:23- Right, then they'd crawl across, yes.- Or occasionally...
0:12:23 > 0:12:24But you're well protected.
0:12:24 > 0:12:27- I am well protected, yes. - I'm less well protected.
0:12:28 > 0:12:32'If, unfortunately, you are infested you will probably have
0:12:32 > 0:12:35'10 to 20 live lice on your head.
0:12:36 > 0:12:38'But there can be hundreds.'
0:12:40 > 0:12:42- So, a good collection of head lice here?- Yeah.
0:12:42 > 0:12:44- They're scuttling around. - There's some young adults,
0:12:44 > 0:12:47- there's some babies that have just hatched so...- They're pretty big. - Yeah.
0:12:47 > 0:12:50- What I'd love you to do then is infect me, please.- OK.
0:12:50 > 0:12:52This is rather unusual, not your normal line of business.
0:12:52 > 0:12:54I can do that.
0:12:55 > 0:12:58'Head lice can only survive by drinking human blood,
0:12:58 > 0:13:01'which they do several times a day.
0:13:02 > 0:13:06'To keep these lice alive, I need to feed them on me.'
0:13:07 > 0:13:09..this could be my first.
0:13:09 > 0:13:12'These little pots should keep them safe while allowing them
0:13:12 > 0:13:14'to bite me.'
0:13:14 > 0:13:15Thank you very much for infecting me.
0:13:15 > 0:13:17You're welcome!
0:13:18 > 0:13:21There aren't many people who would be pleased to hear
0:13:21 > 0:13:24that their visitor is infested with lice.
0:13:24 > 0:13:26- Hi, James. - Hello.
0:13:26 > 0:13:28- I come bearing gifts. - Oh, excellent.
0:13:28 > 0:13:31But James Logan is clearly delighted.
0:13:31 > 0:13:33Let's have a look. Oh, that's brilliant.
0:13:33 > 0:13:35'James studies lice and other parasites at
0:13:35 > 0:13:39'the London School of Hygiene and Tropical Medicine.'
0:13:39 > 0:13:41- OK, so here we have one.- Uh-hm.
0:13:41 > 0:13:44- And shall we put it on your arm? - Why not?
0:13:44 > 0:13:46- See if he scurries around?- Yeah, so I'm...
0:13:46 > 0:13:50'Using a handheld microscope, we can study my head louse
0:13:50 > 0:13:51'as it settles in.'
0:13:53 > 0:13:55But you can really see the blood here, can't you?
0:13:55 > 0:13:57- So this one's quite recently fed. - Yeah.
0:13:57 > 0:13:58On your blood.
0:13:58 > 0:14:02But you can see it being sort of pumped down here through its gut.
0:14:03 > 0:14:07'Like all lice the head louse has retractable mouth parts that
0:14:07 > 0:14:09'can puncture my skin.
0:14:10 > 0:14:14'But what really surprised me is how active they can be.'
0:14:15 > 0:14:19- It's going so fast I can't keep up with it. - This one is quite active, isn't it?
0:14:19 > 0:14:22It certainly is! Look at that, it's gone right to the last hair on your arm!
0:14:22 > 0:14:26- Oh, yeah.- But look at it clinging on to the edge of that hair with... - Yeah.
0:14:26 > 0:14:28- ..with two of its claws. - Look at those claws...
0:14:28 > 0:14:29And it's got another claw out like this.
0:14:29 > 0:14:33I wonder whether it's just sat waiting for another host to come past.
0:14:33 > 0:14:38So that, if I were to rub my head against yours, it would just grab a hold of your hair
0:14:38 > 0:14:40with the other claws and it would be across in an instant, would it?
0:14:40 > 0:14:42Exactly right, yeah.
0:14:42 > 0:14:45It's on your arm, which is not the right place for a head louse.
0:14:45 > 0:14:47So it is probably sitting there thinking,
0:14:47 > 0:14:50"I'm not in the right place, this isn't the right environment
0:14:50 > 0:14:53"and I want to be somewhere where there's more hair."
0:14:56 > 0:15:00'Head lice are well adapted to manoeuvring around the human scalp,
0:15:00 > 0:15:03'their claws the right size for the hairs on our head.
0:15:06 > 0:15:11'But there's another species that makes its home on human hair.
0:15:11 > 0:15:12'The pubic louse.
0:15:14 > 0:15:19'The good news is that pubic lice are increasingly hard to find.
0:15:19 > 0:15:23'When we did finally get hold of these live specimens
0:15:23 > 0:15:24'I was out of the country.
0:15:24 > 0:15:28'But one of my production team BRAVELY volunteered to let them
0:15:28 > 0:15:30'feed on her skin.
0:15:30 > 0:15:32'And filmed this unique footage.'
0:15:34 > 0:15:36And the first thing that you notice is it looks like a crab
0:15:36 > 0:15:39and that's why they get that name, crab lice.
0:15:39 > 0:15:42They're very crab-like, their bodies are much more sort of rounded
0:15:42 > 0:15:45and they're much smaller as well than head lice.
0:15:45 > 0:15:49- What it's designed to do is to live on public hair.- Hmm.
0:15:49 > 0:15:52And pubic hair tends to be spread slightly further apart than
0:15:52 > 0:15:54the hair that you get on your head.
0:15:54 > 0:15:56And so it's designed like this,
0:15:56 > 0:15:59so that it can reach across and grab on to hairs that are further apart.
0:15:59 > 0:16:02- And the hooks are a bit bigger as well, you'll notice.- Hmm.
0:16:02 > 0:16:06And that's because pubic hair tends to be thicker than most hair
0:16:06 > 0:16:07that you get on your head.
0:16:08 > 0:16:12When scientists began to study pubic lice
0:16:12 > 0:16:15they discovered something quite startling.
0:16:15 > 0:16:19That our pubic lice are closely related to
0:16:19 > 0:16:21lice found on another animal.
0:16:21 > 0:16:23One of the great apes.
0:16:25 > 0:16:28It's thought that they came from gorillas originally and you can see
0:16:28 > 0:16:32the sort of divergence between the species that are found on gorillas
0:16:32 > 0:16:35today and the species that we get on us around three million years ago.
0:16:35 > 0:16:38OK, so does that imply that we had sex, or at least our remote ancestors
0:16:38 > 0:16:40had sex with gorillas three million years ago?
0:16:40 > 0:16:45I think that's probably quite a difficult one to answer, but it's
0:16:45 > 0:16:49probably more likely that we shared a sort of environment with gorillas.
0:16:49 > 0:16:51We may have even killed them to eat them
0:16:51 > 0:16:52as bush meat for example, as well.
0:16:52 > 0:16:56So, certainly we came into contact with gorillas.
0:16:57 > 0:17:00Other apes like gorillas and chimpanzees
0:17:00 > 0:17:02have a single species of louse.
0:17:04 > 0:17:07But when our ancestors lost thick body hair and
0:17:07 > 0:17:11gained pubic hair we created two separate islands of hair.
0:17:11 > 0:17:14Providing a niche for gorilla lice.
0:17:14 > 0:17:17All of which suggests we became relatively hairless
0:17:17 > 0:17:20at least three million years ago.
0:17:22 > 0:17:27Having two species of lice is remarkable enough but humans
0:17:27 > 0:17:33actually have three and the third species also has a secret to reveal.
0:17:33 > 0:17:36There's another type of louse called the body louse.
0:17:36 > 0:17:40And I've got an example here to show you what an infestation looks like.
0:17:40 > 0:17:44And this, remarkably, is highly adapted to clothing.
0:17:44 > 0:17:47This was actually... I believe this was from a homeless person.
0:17:47 > 0:17:50- Yeah.- Who had a very heavy infestation.- Oh.
0:17:50 > 0:17:53And I think this...it looks like trousers to me with a seam here.
0:17:53 > 0:17:56And this is exactly where they lay their eggs.
0:17:56 > 0:17:58- Ah!- You can see them... - That is revolting.
0:17:58 > 0:17:59- And those are eggs.- Right.
0:17:59 > 0:18:01So that is a massive infestation,
0:18:01 > 0:18:04- it makes you feel quite itchy, doesn't it?- It does.
0:18:05 > 0:18:08'The body louse behaves very differently to the head louse.'
0:18:08 > 0:18:10So, if we have a look at the lice...
0:18:10 > 0:18:14'But if you compare their body shapes you see something significant.'
0:18:14 > 0:18:17- OK, so the one on the right, the dark one, is my friend the head louse?- Yeah.
0:18:17 > 0:18:20He's looking rather dead at the moment, I have to say.
0:18:20 > 0:18:23Not looking good, and the one on the left, they're the body lice, are they?
0:18:23 > 0:18:26That's exactly right. And what you can see is that they look remarkably similar.
0:18:26 > 0:18:27And when you look at their DNA
0:18:27 > 0:18:31what you find is that they diverged around 100, 200 thousand years ago.
0:18:31 > 0:18:34So not actually that long ago in sort of evolutionary terms,
0:18:34 > 0:18:37but enough to make them different species. And so that tells us
0:18:37 > 0:18:40something about when we as humans started to wear clothing.
0:18:40 > 0:18:43So before that, the idea is humans are naked?
0:18:43 > 0:18:45Then they start to wear clothes, maybe a head louse
0:18:45 > 0:18:49- kind of drops down, thinks it's actually quite a nice place to live? - Yeah.
0:18:49 > 0:18:52And begins to evolve, a whole new species starts to evolve on clothes?
0:18:52 > 0:18:54- Oh. - Exactly right, yeah.
0:18:54 > 0:18:56So it's quite incredible to think that you can look at
0:18:56 > 0:19:00the louse to find out more about our own ancestry and our own evolution.
0:19:02 > 0:19:07So both the lice and the tapeworm have evolved to live on the human body.
0:19:08 > 0:19:10Changing their bodies and
0:19:10 > 0:19:12behaviour to exploit our anatomy.
0:19:17 > 0:19:20But evolutionary pressure works both ways -
0:19:20 > 0:19:26over time we have also been shaped by the parasites we carry.
0:19:26 > 0:19:28What do you think of those worms?
0:19:28 > 0:19:29WOMAN: Horrible.
0:19:29 > 0:19:30Gut worms.
0:19:30 > 0:19:32Oh!
0:19:32 > 0:19:34- WOMAN:- Would they make you feel bad?
0:19:34 > 0:19:36Or would you be happy to have those in your tummy?
0:19:36 > 0:19:39- Feel bad.- A bit bad? Yeah, would it make you feel a bit sick?
0:19:39 > 0:19:42'Val Curtis believes that parasites have shaped some
0:19:42 > 0:19:44'fundamental human instincts.'
0:19:44 > 0:19:46They get hungry, they can smell you.
0:19:46 > 0:19:48They can smell your warmth and they come out...
0:19:48 > 0:19:52maybe 20 minutes they spend sucking on you and you don't even notice they're there.
0:19:52 > 0:19:53And then as soon...
0:19:53 > 0:19:56'To demonstrate, we set up a street stall,
0:19:56 > 0:19:59'to see what sort of reactions we'll get from parasites.'
0:20:00 > 0:20:03Which of these things do you find most disgusting?
0:20:03 > 0:20:05Oh!
0:20:05 > 0:20:06What about this...?
0:20:06 > 0:20:09- What does it do. - That fly lays eggs on your skin
0:20:09 > 0:20:10and then it grows under your skin.
0:20:10 > 0:20:12< That's disgusting.
0:20:12 > 0:20:13ALL: Eww!
0:20:13 > 0:20:15That's how big they are.
0:20:15 > 0:20:18Can you imagine having one of those in your leg or in your head?
0:20:20 > 0:20:25'Val believes we evolved the disgust response...
0:20:25 > 0:20:26'to prevent us
0:20:26 > 0:20:30'being infected with disease- causing organisms like parasites.'
0:20:30 > 0:20:32What about this?
0:20:33 > 0:20:36Most people think of disgust as just being a bad thing,
0:20:36 > 0:20:38but actually disgust is incredibly good for you.
0:20:38 > 0:20:42Let's just imagine one of your great-great-great ancestors
0:20:42 > 0:20:45and that...maybe that great ancestor wasn't squeamish
0:20:45 > 0:20:48and maybe that great ancestor went "Oh, oh, my goodness, a poo, look."
0:20:48 > 0:20:49HE LAUGHS
0:20:49 > 0:20:53- "There's a bit of food left in there, yum, yum." - That's not, is it, it's plastic...
0:20:53 > 0:20:56So maybe, you know, nibble, nibble, nibble on the poo.
0:20:56 > 0:20:58Now, your great ancestor who does that,
0:20:58 > 0:20:59is he going to find a wife easily?
0:20:59 > 0:21:02- LAUGHTER - Not terribly well, right? - Yeah.
0:21:02 > 0:21:04How about his children and his grandchildren,
0:21:04 > 0:21:07are they going to be healthy? No, they're not.
0:21:07 > 0:21:10So it's a fair bet that every one of your ancestors had a good,
0:21:10 > 0:21:13healthy sense of squeamishness and, as a result,
0:21:13 > 0:21:16you have their squeamishness genes.
0:21:16 > 0:21:19And it grows into something a bit like that.
0:21:20 > 0:21:22'To reduce the risk of infection
0:21:22 > 0:21:24'we instinctively find parasites
0:21:24 > 0:21:27'and anything to do with them disgusting.'
0:21:28 > 0:21:29- The head lice.- Ew!
0:21:29 > 0:21:31How does that make you feel?
0:21:31 > 0:21:33- We've just had our hair cut! - Oh, gosh!
0:21:33 > 0:21:34LAUGHTER
0:21:36 > 0:21:40'The most likely source of parasitic infections is other human beings.
0:21:40 > 0:21:44'For a social species like us, that presents a real dilemma.'
0:21:46 > 0:21:48'I want to be your friend, you know.
0:21:48 > 0:21:50'I want to learn something from you.'
0:21:50 > 0:21:53And I want to exchange with you and all of us want to do that.
0:21:53 > 0:21:54That's the human way of life.
0:21:54 > 0:21:58We're a deeply social species, we co-operate, we exchange.
0:21:58 > 0:22:00But you're at the same time a bag of parasites.
0:22:00 > 0:22:02Yeah.
0:22:02 > 0:22:04You're absolutely stuffed full of parasites.
0:22:04 > 0:22:06So I don't want to get too close to you.
0:22:06 > 0:22:09I don't want to be within spitting distance of you.
0:22:09 > 0:22:11And in fact, if you look at the way people
0:22:11 > 0:22:14behave around the world, we don't sit too close to each other.
0:22:14 > 0:22:16We're very careful about wearing clean clothes.
0:22:16 > 0:22:19We're very careful about not being smelly and not being...
0:22:19 > 0:22:22- not sharing our bodily emanations. - Yeah.- That's good manners.
0:22:22 > 0:22:25So you think that manners are a product of parasites?
0:22:25 > 0:22:27Parasites teach us manners
0:22:27 > 0:22:30and manners are fundamental to what make us human.
0:22:30 > 0:22:31Hurrah for the parasites?
0:22:33 > 0:22:37Oddly, my reaction to having a tapeworm infection is more
0:22:37 > 0:22:38curiosity than disgust.
0:22:39 > 0:22:42To find out more I met up with the UK's
0:22:42 > 0:22:45leading tapeworm expert.
0:22:45 > 0:22:47OK, it's about a week now.
0:22:47 > 0:22:49It's going to be about that long?
0:22:49 > 0:22:51I should say so, yes. I'm taking that as
0:22:51 > 0:22:53a calculation
0:22:53 > 0:22:56that after 10 weeks I know it'll be 3m.
0:22:56 > 0:22:58But we don't really know now, of course.
0:22:58 > 0:23:01We're just assuming the three cysts you took were viable,
0:23:01 > 0:23:03and they looked viable, at least two of them, I think.
0:23:03 > 0:23:07- How would you tell, in any way? - I don't think you will...
0:23:07 > 0:23:08No, I think...
0:23:08 > 0:23:11if you didn't know that you were infected, you wouldn't know.
0:23:11 > 0:23:13No. I have these moments when I think...
0:23:13 > 0:23:14I've got a little gut rot
0:23:14 > 0:23:16but then I think...
0:23:16 > 0:23:18The fact that you know you're infected
0:23:18 > 0:23:20means that you might feel like you've a bit more
0:23:20 > 0:23:23of a craving for beer or for carb...or for chocolate.
0:23:23 > 0:23:25Or something that you like, perhaps chocolate?
0:23:25 > 0:23:27No, absolutely, that's what I said to my wife.
0:23:27 > 0:23:30And people do report that, you know, they do eat more.
0:23:30 > 0:23:34A tapeworm has no digestive system of its own.
0:23:34 > 0:23:36No mouth, no stomach.
0:23:36 > 0:23:38No anus.
0:23:38 > 0:23:39Instead...
0:23:39 > 0:23:41it absorbs the food I've already digested.
0:23:43 > 0:23:47Its skin is very similar to the inner surface of my intestines.
0:23:47 > 0:23:50With lots of tiny bumps called villi.
0:23:51 > 0:23:54These increase the surface area of the tapeworm
0:23:54 > 0:23:56so it can absorb nutrients
0:23:56 > 0:23:57directly through its skin.
0:23:59 > 0:24:01So, what should I feed it?
0:24:01 > 0:24:03It can take up carbohydrates
0:24:03 > 0:24:05and glycoproteins very effectively.
0:24:05 > 0:24:07- It likes them? - It likes them.
0:24:07 > 0:24:11'Anything rich in carbs will be good for the worm.'
0:24:11 > 0:24:14In fact that does remind me of the type of tapeworm that you
0:24:14 > 0:24:16might have.
0:24:16 > 0:24:20- Yes, the proportions are fairly accurate actually.- Yeah.
0:24:20 > 0:24:23That's going to be about the width of the worm after about 10 weeks.
0:24:23 > 0:24:24Yeah.
0:24:24 > 0:24:26And certainly, if it's left longer than that,
0:24:26 > 0:24:29going into months it will get about twice the width.
0:24:29 > 0:24:31So the worm likes carbohydrate?
0:24:31 > 0:24:33Well, I think it would, yes.
0:24:33 > 0:24:34HE SLURPS
0:24:34 > 0:24:37Certainly that's good for a growing tapeworm, Michael.
0:24:37 > 0:24:38HE LAUGHS
0:24:38 > 0:24:40If you have a bowl of that a day.
0:24:40 > 0:24:42- Yes, I'll give it a go.- It'll grow.
0:24:42 > 0:24:45- Good and strong.- Probably to good proportions, yeah, yeah.
0:24:47 > 0:24:51Head lice are rarely more than an irritation.
0:24:51 > 0:24:54And so far my tapeworm is proving very benign.
0:24:55 > 0:24:58It's a strategy that makes sense.
0:24:58 > 0:25:01By causing minimal harm the parasites help to ensure
0:25:01 > 0:25:03their host's survival.
0:25:06 > 0:25:09But not all parasites take this approach.
0:25:09 > 0:25:12An example of a lethal parasite is this one.
0:25:15 > 0:25:18It's a microscopic, single-celled parasite...
0:25:19 > 0:25:21..called plasmodium.
0:25:23 > 0:25:27It causes malaria, a disease that kills more than
0:25:27 > 0:25:29half a million people a year.
0:25:33 > 0:25:37I want to find out why it is so deadly.
0:25:37 > 0:25:40So, I've come to the National Institute of Medical Research,
0:25:40 > 0:25:43where they will infect my blood with plasmodium.
0:25:46 > 0:25:49- So, we've got a sample of my blood here looking...- Yes.
0:25:49 > 0:25:50..nice and red, healthy?
0:25:50 > 0:25:52Yes, yes, yes, and warm, straight out of your body.
0:25:52 > 0:25:55- MICHAEL LAUGHS - Hi, there.
0:25:55 > 0:25:56- Hi.- I won't shake hands.
0:25:56 > 0:25:59- LAUGHTER Thanks very much, Fiona.- OK.
0:25:59 > 0:26:02- Well, I guess we're not allowed in that room. - Oh, that's right, yes.
0:26:02 > 0:26:04So, what's Fiona up to at the moment, then?
0:26:04 > 0:26:06So, what's she going to do now is she's going to take
0:26:06 > 0:26:08a sample of plasmodium falciparum.
0:26:08 > 0:26:12So this is the most dangerous form of the parasite that causes
0:26:12 > 0:26:15most deaths in, for example, Sub-Saharan Africa.
0:26:15 > 0:26:16We've grown this in the lab.
0:26:16 > 0:26:19We can culture it continuously in human red cells
0:26:19 > 0:26:23and she's now going to take a sample of those parasites and inoculate them into your blood.
0:26:23 > 0:26:28Right, and I must admit, out of all the diseases, malaria is high on the ones I want to avoid!
0:26:28 > 0:26:30Well, you should try to avoid it if you can.
0:26:32 > 0:26:35To see the malaria parasite multiplying in my blood
0:26:35 > 0:26:39they're using a new photographic time-lapse technique.
0:26:39 > 0:26:42Although I have treated people with malaria I have never seen
0:26:42 > 0:26:46the malaria parasite in action before.
0:26:46 > 0:26:49Very intrigued to see what happens to my blood,
0:26:49 > 0:26:52but what is chilling is thinking that out there in the world
0:26:52 > 0:26:55mosquitoes are infecting children.
0:26:55 > 0:26:57And a thousand children every day are dying.
0:26:57 > 0:27:00And that's why this sort of work is so important.
0:27:02 > 0:27:06Here it is, my blood infected with the deadly malaria parasite.
0:27:07 > 0:27:10So, the parasite goes through this life cycle in the infected
0:27:10 > 0:27:12red cell, it infects...
0:27:12 > 0:27:15'Once the infection is complete, Mike Blackman shows me
0:27:15 > 0:27:20'the finished movie which reveals how the parasite kills its host.'
0:27:20 > 0:27:25- The little white blobs here, they are the parasite, is that right? - They are the parasite, yes.
0:27:25 > 0:27:29So a single merozoite, this invasive form of the parasite.
0:27:29 > 0:27:33Binds to a red cell, invades it, grows within it.
0:27:33 > 0:27:35Digests the haemoglobin of the red cell.
0:27:35 > 0:27:39This is the red protein that is used to carry oxygen by your red cells.
0:27:39 > 0:27:45- And then eventually forms around about 16 to 32 daughter merozoites. Right.- Yes.
0:27:45 > 0:27:49- So they're... Oh, wow, look at that one go.- Yes.- That one just exploded, didn't it?
0:27:49 > 0:27:52- That's right... So that... - And suddenly you're seeing lots and lots...
0:27:52 > 0:27:54- Oh, wow, they're really going. - Yes, yeah.
0:27:56 > 0:28:00The whole thing is destroyed in a single, very rapid process.
0:28:00 > 0:28:04The merozoites are released and they immediately invade a new cell and these things just...
0:28:04 > 0:28:06I had no idea it was going to be that violent.
0:28:06 > 0:28:08I mean, that was utterly destroyed.
0:28:09 > 0:28:14'Each explosion obliterates one of my oxygen-carrying red blood cells
0:28:14 > 0:28:19'and releases new parasites into my blood to infect yet more cells.'
0:28:21 > 0:28:23And suddenly you go on from a situation where
0:28:23 > 0:28:27there are relatively few, but they're everywhere, aren't they? They're just swarming.
0:28:27 > 0:28:31I mean, you do appreciate just what a terrible thing they're doing inside you.
0:28:31 > 0:28:35Yeah, if this were going on inside you, you'd be in a pretty poor way.
0:28:35 > 0:28:38'You soon become anaemic from lack of blood cells
0:28:38 > 0:28:40'and the debris from all those shattered cells
0:28:40 > 0:28:44'can block your blood vessels, which, in turn, can be fatal.'
0:28:45 > 0:28:48So our interest here is exactly what...how this goes on.
0:28:48 > 0:28:50How does the parasite actually do that?
0:28:50 > 0:28:52We don't really know.
0:28:52 > 0:28:54The parasite is very, very good, very smart.
0:28:54 > 0:28:58It's evolved with its human hosts for a long time.
0:28:58 > 0:29:00MOSQUITOES BUZZ
0:29:00 > 0:29:02For all parasites,
0:29:02 > 0:29:06getting into a new host is essential for their long-term survival.
0:29:06 > 0:29:10That's particularly important for parasites that kill their hosts.
0:29:11 > 0:29:16For plasmodium the key to infecting a new human host is this -
0:29:16 > 0:29:17the mosquito.
0:29:20 > 0:29:24New research has revealed that once the malaria parasite
0:29:24 > 0:29:26infects a mosquito
0:29:26 > 0:29:29it actually sharpens its sense of smell.
0:29:31 > 0:29:36Making the mosquito more likely to seek out and bite a human being.
0:29:37 > 0:29:42And as it sucks our blood it injects the parasite into us.
0:29:45 > 0:29:48But there are other parasites that use even more
0:29:48 > 0:29:52extraordinary strategies for getting into a new host.
0:29:55 > 0:29:58Like this parasite - a kind of flatworm.
0:30:03 > 0:30:07Although it spends the first part of its life cycle in a snail,
0:30:07 > 0:30:11it needs to infect a bird in order to reproduce.
0:30:12 > 0:30:16To make sure the snail becomes bird food, it crawls into the
0:30:16 > 0:30:22snail's eye and the eye swells up to resemble a tasty caterpillar.
0:30:25 > 0:30:28What's more, the worm wriggles to put on
0:30:28 > 0:30:30a literally eye-catching show.
0:30:31 > 0:30:34It also changes the snail's behaviour.
0:30:34 > 0:30:39Usually, a snail hides in dark places,
0:30:39 > 0:30:43but an infected snail will happily come out into the light.
0:30:44 > 0:30:48This massively increases its chance of being eaten by a bird.
0:30:48 > 0:30:52Bad news for the snail, but great for the parasite.
0:30:56 > 0:31:00I do find it particularly chilling, the way that the snail parasite
0:31:00 > 0:31:03hijacks the brain of its host,
0:31:03 > 0:31:06and makes it give up its life for the sake of the parasite.
0:31:06 > 0:31:09But this is not an isolated example.
0:31:09 > 0:31:13There are other parasites that adopt similar strategies.
0:31:13 > 0:31:16This is toxoplasma gondii.
0:31:17 > 0:31:22It's a microscopic parasite that looks like a wriggling comma,
0:31:22 > 0:31:26and it's possibly the most successful parasite on Earth.
0:31:28 > 0:31:33Its life always starts in a cat, where the parasite breeds.
0:31:36 > 0:31:43An infected cat passes oocysts, tiny capsules containing the parasites.
0:31:43 > 0:31:46The oocysts can survive on the ground for months,
0:31:46 > 0:31:52waiting to be eaten by a rat or mouse, which then becomes a carrier.
0:31:56 > 0:31:59But for the life cycle to complete, the parasite has to get
0:31:59 > 0:32:05back into a cat, which means the rat has to be caught and eaten by a cat.
0:32:10 > 0:32:15So toxoplasma has to encourage its mouse host to commit suicide.
0:32:17 > 0:32:22A few years ago, Joanne Webster set up a series of elegant experiments
0:32:22 > 0:32:27to see how toxoplasma might go about altering a rodent's behaviour,
0:32:27 > 0:32:30making it more likely to get eaten.
0:32:31 > 0:32:35She placed infected and uninfected rats in a chamber,
0:32:35 > 0:32:39where she had liberally doused one corner with cat urine.
0:32:39 > 0:32:42And we simply plop the rat in and let them
0:32:42 > 0:32:44tootle about over each four hour, ten hour night,
0:32:44 > 0:32:47and simply watch where they went.
0:32:47 > 0:32:49Alice here is an uninfected female.
0:32:49 > 0:32:52She smelt the cat area and she shot off here.
0:32:52 > 0:32:56- She seems to be avoiding the cat-smelling one.- Absolutely.
0:32:56 > 0:32:58- She's hanging around in different areas.- Yeah.
0:32:58 > 0:33:01'Putting in a rat infected with toxoplasma
0:33:01 > 0:33:03'produced a very different result.'
0:33:03 > 0:33:05This is Felix here.
0:33:05 > 0:33:07He's infected, and he's actually in the cat zone.
0:33:07 > 0:33:09Which is actually not what you'd expect.
0:33:09 > 0:33:11A normal mouse or a normal rat would run.
0:33:11 > 0:33:14- Smell it and absolutely hide. - But he seems to be just enjoying it.
0:33:14 > 0:33:17He's out, he's active, he's tootling about.
0:33:17 > 0:33:21So presumably, if you're the parasite what you want, you want
0:33:21 > 0:33:24the rat, Felix the rat, to hang around near where cats are,
0:33:24 > 0:33:26- because you want to be eaten.- Yes.
0:33:28 > 0:33:34Joanne had shown that infected rats are attracted to cat smells.
0:33:34 > 0:33:38They're also generally more fearless and have slower reaction times.
0:33:38 > 0:33:43All these factors made them far more likely to become cat victims.
0:33:44 > 0:33:48What is rather disturbing is that this parasite, toxoplasma,
0:33:48 > 0:33:50also infects us.
0:33:50 > 0:33:53Around a quarter of the British population are infected
0:33:53 > 0:33:55without knowing it.
0:33:55 > 0:33:57You can catch it from cat litter.
0:33:58 > 0:34:00Or infected soil.
0:34:02 > 0:34:07Or from eating undercooked meat from an animal that was itself infected.
0:34:11 > 0:34:15Since toxoplasma can manipulate rat brains,
0:34:15 > 0:34:18can the parasite also change our behaviour?
0:34:18 > 0:34:21There's intriguing evidence that it can.
0:34:24 > 0:34:27Scientists have analysed blood samples from people involved in
0:34:27 > 0:34:31traffic accidents, and they found they are more than twice as likely
0:34:31 > 0:34:34to be infected with toxoplasma than the average.
0:34:35 > 0:34:38The claim is that infected people are slower to react
0:34:38 > 0:34:41and take greater risks.
0:34:41 > 0:34:43Just like infected rats.
0:34:47 > 0:34:49I'm a keen cyclist,
0:34:49 > 0:34:53and I admit I've had more than my fair share of accidents.
0:34:56 > 0:35:01A while ago I was in collision with a white van and I got knocked out.
0:35:01 > 0:35:04More recently, I hit the kerb while cycling.
0:35:04 > 0:35:09Came off, fractured my ribs and also broke my elbow.
0:35:09 > 0:35:13Now, my family thinks it's because I'm careless, reckless.
0:35:15 > 0:35:19But could it be it's because I'm infected with toxoplasma?
0:35:21 > 0:35:26I've had a blood test to find out, and Joanne has the results.
0:35:26 > 0:35:27What do you feel you are?
0:35:27 > 0:35:30I would be, I guess, surprised if I wasn't,
0:35:30 > 0:35:35in the sense that I am aware that sometimes I do remarkably
0:35:35 > 0:35:38stupid things and in retrospect I wonder why I've done them.
0:35:38 > 0:35:41- So you could have a few parasites. - I wonder if there are a few.
0:35:41 > 0:35:43I am dying to find out, I must admit.
0:35:48 > 0:35:50And you are negative.
0:35:50 > 0:35:54I am negative. OK, so I can't blame the parasite for my risky behaviour,
0:35:54 > 0:35:56I just am. It's quite scary isn't it,
0:35:56 > 0:35:59the thought of a parasite manipulating your behaviour?
0:35:59 > 0:36:02Absolutely, and I think the fact that it almost brings on the concept
0:36:02 > 0:36:06of free will, because how much of your behaviour are you expressing
0:36:06 > 0:36:09is yours and how much is it the parasite within you?
0:36:09 > 0:36:12Does toxoplasma change human behaviour?
0:36:12 > 0:36:15At the moment, there's no direct evidence.
0:36:16 > 0:36:20If there is an effect it will be, in the vast majority of cases,
0:36:20 > 0:36:21very subtle.
0:36:22 > 0:36:26But the fact that it can alter a rat's behaviour is nothing
0:36:26 > 0:36:28short of remarkable.
0:36:28 > 0:36:31It shows just what extraordinary abilities evolution
0:36:31 > 0:36:36has equipped parasites with to ensure they spread to a new host.
0:36:40 > 0:36:43Tapeworm may not go in for mind control...
0:36:44 > 0:36:47..but they do have some ingenious tricks.
0:36:47 > 0:36:50At two weeks old, my tapeworm is still a harmless youngster.
0:36:50 > 0:36:52But when it reaches maturity,
0:36:52 > 0:36:55it will start doing something rather unpleasant.
0:36:57 > 0:37:02That is the adult tapeworm, and this tapeworm is exactly ten weeks old.
0:37:02 > 0:37:06We know that because it was from a previous volunteer.
0:37:08 > 0:37:14By ten weeks, the mature segments are full of eggs, and they break off
0:37:14 > 0:37:16from the tapeworm and emerge from your body...
0:37:19 > 0:37:21..ready to infect a cow.
0:37:24 > 0:37:28So it comes out of your bottom?
0:37:28 > 0:37:29And it can actually crawl out?
0:37:29 > 0:37:34Unfortunately, yes. Psychologically, that upsets quite a few people
0:37:34 > 0:37:36that are infected.
0:37:36 > 0:37:38Yeah, it can come out under its own steam, as it were.
0:37:38 > 0:37:42So, once the segment is in the rectum, it'll move around and that
0:37:42 > 0:37:46causes a sort of a strange fluttering sensation, and then
0:37:46 > 0:37:51it will move through the rectum, through the anus, it'll crawl around
0:37:51 > 0:37:55between the buttocks, down the legs and out onto the floor.
0:37:55 > 0:37:57- Right.- Can be embarrassing.
0:37:57 > 0:37:59It might surprise my wife or friends.
0:37:59 > 0:38:01I think it probably would, yes.
0:38:01 > 0:38:05Often, the first time someone realises they are infected
0:38:05 > 0:38:08is when a segment crawls out.
0:38:08 > 0:38:10One of the reasons I'm doing this self experiment is to help
0:38:10 > 0:38:12Phil change that.
0:38:12 > 0:38:16I'm going to be sending you some sort of faeces samples
0:38:16 > 0:38:18on a weekly basis. What do you want them for?
0:38:18 > 0:38:25Well, that's to... For us to help develop or refine a diagnostic test.
0:38:25 > 0:38:28Even though this worm is very big, people that are infected
0:38:28 > 0:38:31aren't always aware they're infected, especially early on in the
0:38:31 > 0:38:35infection, and we can actually take a faecal sample, a stool sample,
0:38:35 > 0:38:39we can test it for presence of tapeworm carbohydrates.
0:38:39 > 0:38:42We can also test it for tapeworm DNA, and obviously,
0:38:42 > 0:38:45if we find that, it suggests that the carrier
0:38:45 > 0:38:47does have a tapeworm infection.
0:38:49 > 0:38:53Phil's research is important because tapeworm can be very dangerous.
0:38:54 > 0:38:58Beef tapeworm like mine are relatively benign.
0:39:00 > 0:39:02But other types, like the pork tapeworm,
0:39:02 > 0:39:06can form cysts in the brain which can kill.
0:39:07 > 0:39:09So far, I've encountered nothing
0:39:09 > 0:39:12but the downside of being infected by a parasite.
0:39:12 > 0:39:13Yet, as we've seen,
0:39:13 > 0:39:16parasites come equipped with a remarkable range of skills,
0:39:16 > 0:39:21and sometimes we can use their skills to our advantage.
0:39:24 > 0:39:28This is a leech. For thousands of years, they were used to treat
0:39:28 > 0:39:32everything from skin diseases to fevers.
0:39:32 > 0:39:35Those cures almost certainly did more harm than good.
0:39:36 > 0:39:41But in the right hands, the leech can be a useful surgical assistant.
0:39:42 > 0:39:46Ian Whitaker is a reconstructive surgeon who is pioneering
0:39:46 > 0:39:48the modern use of leeches.
0:39:49 > 0:39:53I normally use them on extremities. For example, a finger,
0:39:53 > 0:39:56or in rare instances, a nose or an ear.
0:39:56 > 0:39:58You're re-attaching the end of a finger or something?
0:39:58 > 0:39:59Yes, that's right.
0:39:59 > 0:40:03It is technically possible, with microsurgery, to attach the artery,
0:40:03 > 0:40:05so you've got blood flowing in,
0:40:05 > 0:40:07but the blood flowing out via the veins -
0:40:07 > 0:40:09- it's much more difficult. - Things start to swell up?
0:40:09 > 0:40:11Yes, that's exactly right, that's the basis of it.
0:40:11 > 0:40:14And the leeches are quite good at controlling the flow, are they?
0:40:14 > 0:40:17Yes, it's almost as if they're custom-made.
0:40:17 > 0:40:21You know, they remove a fairly predictable amount of blood
0:40:21 > 0:40:23and it's self-contained.
0:40:23 > 0:40:26They promote bleeding after their removal as well.
0:40:28 > 0:40:31'It's the ability to remove blood that makes the leech
0:40:31 > 0:40:38'so useful in surgery. But how much blood can a leech consume?
0:40:38 > 0:40:40'Well, there's one way to find out.
0:40:41 > 0:40:45'First, we have to see how much the hungry leech weighs.'
0:40:45 > 0:40:48- 0.4.- That's perfect.
0:40:48 > 0:40:50- OK.- So shall we put it on?
0:40:50 > 0:40:52- Are you ready?- I'm ready, yeah.
0:40:54 > 0:40:56He only needs to feed once a year.
0:40:56 > 0:40:59It takes about three months to digest a blood meal.
0:41:01 > 0:41:06I can feel it, yeah. I can definitely something going on there.
0:41:06 > 0:41:09At the moment, it's attached by its head end where the jaws
0:41:09 > 0:41:11and teeth would have made a hole.
0:41:13 > 0:41:18Here, you can see how the leech's triple-jawed mouth is able to
0:41:18 > 0:41:22cut through a membrane. That's what it's doing to my skin.
0:41:25 > 0:41:29Once it's made a hole, it can start feeding on my blood.
0:41:32 > 0:41:35It sucks by contracting its muscles
0:41:35 > 0:41:38in a rhythmic movement called peristalsis.
0:41:40 > 0:41:44Its digestive tract visible here in red can expand
0:41:44 > 0:41:46to hold huge quantities of blood.
0:41:49 > 0:41:53And its saliva contains proteins that help the blood keep flowing.
0:41:56 > 0:42:00And it will be injecting some fairly ingenious things,
0:42:00 > 0:42:02hirudin is probably the most important,
0:42:02 > 0:42:06which will prevent it clotting, so it can feed more efficiently.
0:42:06 > 0:42:09It's the most potent anticoagulant known to man.
0:42:09 > 0:42:11How long will I go on bleeding for?
0:42:11 > 0:42:13I think the average is about 12 hours.
0:42:13 > 0:42:18In some instances, it can go up to 48 and 72 in extreme circumstances.
0:42:18 > 0:42:22Oh, God, it's moved. It's come off. Oh, there we go.
0:42:22 > 0:42:24Shall we see how heavy it is?
0:42:24 > 0:42:28Hey, blimey, so it's now seven or eight times heavier.
0:42:28 > 0:42:30That is phenomenal. On you that would be...?
0:42:30 > 0:42:35That's equivalent of me... 115 to 120 stone after one meal.
0:42:35 > 0:42:41By helping blood to flow freely into a newly reattached tissue,
0:42:41 > 0:42:45the leech can save parts of the body that would otherwise die.
0:42:46 > 0:42:50It's a very upsetting injury to lose a finger or a thumb
0:42:50 > 0:42:54and leeches are literally the answer in some cases
0:42:54 > 0:42:56when we can't get any other way to remove the blood,
0:42:56 > 0:42:59so they literally will save people's careers.
0:43:00 > 0:43:04'The growing use of leeches in surgery has led to unusual
0:43:04 > 0:43:06'business opportunities...
0:43:07 > 0:43:09'..leech breeding.
0:43:09 > 0:43:15'I've come to visit the UK's only leech farm.'
0:43:15 > 0:43:17So how many leeches do you have here?
0:43:17 > 0:43:20It's about 35,000 at the moment and it goes up to about 70,000.
0:43:20 > 0:43:25I wouldn't have imagined there was a world demand for 70,000 leeches.
0:43:27 > 0:43:30'So how do you go about farming leeches?'
0:43:31 > 0:43:33These are our breeding chambers.
0:43:33 > 0:43:35The leeches in the wild they would breed in water
0:43:35 > 0:43:40- and they come up into land to lay their eggs.- OK.
0:43:41 > 0:43:44- So this is kind of like a swamp? - Yes, like a swamp.
0:43:44 > 0:43:46You can see there's a freshly-laid cocoon there.
0:43:48 > 0:43:52'Each cocoon contains 25 tiny baby leeches.'
0:43:53 > 0:43:56Oh, they're bit nasty, aren't they?
0:43:56 > 0:44:00- I know, you never tire of the miracle of birth.- Yeah.
0:44:04 > 0:44:09'Most of the leeches here are of the species called Hirudo verbana...
0:44:10 > 0:44:13'..but they also have another significantly larger
0:44:13 > 0:44:16'species of leech that normally feeds
0:44:16 > 0:44:19'on Southeast Asian water buffalo.'
0:44:19 > 0:44:23We tend to use this species mainly for veterinary use.
0:44:23 > 0:44:28Will it bite? Ow! That's unpleasant! That is so unpleasant!
0:44:29 > 0:44:33That really freaked me in ways I wasn't expecting. Eugh!
0:44:36 > 0:44:40Funnily enough, I found that buffalo leech much more disgusting
0:44:40 > 0:44:42than swallowing the tapeworm cyst.
0:44:44 > 0:44:48It's now over three weeks since I did so and the worm should be
0:44:48 > 0:44:52at least a foot long, but it still isn't causing me any problems.
0:44:54 > 0:44:56I haven't noticed any stomach upsets,
0:44:56 > 0:45:01perhaps a little bit more craving for sugar.
0:45:01 > 0:45:03Definitely feel the urge for chocolate and biscuits
0:45:03 > 0:45:05but that's also completely normal.
0:45:05 > 0:45:09So absolutely no side effects.
0:45:09 > 0:45:14Apart from dreaming on one occasion I had all these little things
0:45:14 > 0:45:18coming out of my bottom. Nocturnal emissions!
0:45:18 > 0:45:20But when I woke up I was relieved they weren't there.
0:45:24 > 0:45:29A bacterial gut infection will often cause diarrhoea and stomach pains.
0:45:29 > 0:45:31And yet I feel nothing.
0:45:31 > 0:45:34To find out why, I met up with Helena Helmby,
0:45:34 > 0:45:38who studies how worms interact with our immune system.
0:45:38 > 0:45:40You have a very fine collection of worms here.
0:45:40 > 0:45:42Some of them I recognise.
0:45:42 > 0:45:45Tapeworm... This one I'm less familiar with.
0:45:45 > 0:45:47This is Ascaris, the large roundworm.
0:45:49 > 0:45:51The thing that puzzles me
0:45:51 > 0:45:54is how do these worms evade the human immune system,
0:45:54 > 0:45:56because some of them are huge?
0:45:56 > 0:46:00Yes. And we have a very sophisticated immune system
0:46:00 > 0:46:02which is constantly on watch.
0:46:02 > 0:46:06Guarding against any microorganism 24/7.
0:46:06 > 0:46:10But these large parasites have developed a very sophisticated way
0:46:10 > 0:46:13of dealing with our immune system,
0:46:13 > 0:46:17because they have evolved with us for thousands if not millions of years.
0:46:18 > 0:46:23The worms have evolved mechanisms to dampen down our immune responses.
0:46:23 > 0:46:28By secreting compounds that manipulate our immune system.
0:46:28 > 0:46:34That enables the worms to survive, but may also have benefits for us.
0:46:34 > 0:46:38The worms are allowed to stay because the immune system realises
0:46:38 > 0:46:45that the attack to kill them would be far too dangerous for the host.
0:46:45 > 0:46:49Obviously you don't want a massive inflammatory response in your intestine
0:46:49 > 0:46:51because that would cause a lot of damage to your intestine,
0:46:51 > 0:46:53and that would be dangerous for you as a host.
0:46:53 > 0:46:55So there's a sort of uneasy truce
0:46:55 > 0:46:59between the worm and the host in this case.
0:46:59 > 0:47:00It is extraordinary, isn't it?
0:47:00 > 0:47:03- Yes, absolutely fantastic! - You're a fan?
0:47:03 > 0:47:04Yes, I love them!
0:47:06 > 0:47:11So our bodies have evolved a kind of ceasefire with the worms.
0:47:11 > 0:47:14But in a modern, technological society,
0:47:14 > 0:47:16most people spend their lives worm-free.
0:47:18 > 0:47:21Some scientists believe this may have contributed
0:47:21 > 0:47:23to the rise of allergies.
0:47:25 > 0:47:27It's known as the hygiene hypothesis.
0:47:29 > 0:47:33So you buy the idea that one of the reasons we have this huge surge
0:47:33 > 0:47:37in allergies is because we're just not exposed to as many worms
0:47:37 > 0:47:39or microorganisms as we used to be?
0:47:39 > 0:47:43It's quite possible, because that particular arm of the immune system
0:47:43 > 0:47:46that reacts when we get a strong allergic reaction
0:47:46 > 0:47:48to grass pollen or something like that,
0:47:48 > 0:47:50is also the same arm of immune response
0:47:50 > 0:47:53that we use to fight parasites.
0:47:53 > 0:47:57And if we are now not having these parasites, perhaps that part
0:47:57 > 0:47:59of the immune system doesn't really know what to do with itself
0:47:59 > 0:48:05and it goes off and reacts badly to innocuous allergens
0:48:05 > 0:48:10such as grass pollen or cat dandruff, or whatever it is you're allergic to.
0:48:12 > 0:48:15So perhaps having a worm inside you
0:48:15 > 0:48:19will help control your immune system and keep it in balance.
0:48:19 > 0:48:22It's not proven but the evidence is intriguing.
0:48:23 > 0:48:27There's no doubt that there's been a huge rise in allergies and so-called
0:48:27 > 0:48:31autoimmune diseases as we've got richer and more hygienic.
0:48:34 > 0:48:37And rates of these diseases are highest in countries that
0:48:37 > 0:48:39don't have lots of parasitic worms.
0:48:45 > 0:48:49But what is controversial is that some people are now using worms
0:48:49 > 0:48:51to try and treat their diseases.
0:48:53 > 0:48:55Daniel Heyman has Crohn's.
0:48:57 > 0:49:01It's a debilitating condition in which the immune system
0:49:01 > 0:49:04in the gut starts attacking the body's own tissues.
0:49:04 > 0:49:07I was in Australia, travelling with my parents.
0:49:07 > 0:49:10I was very ill the whole time I was there but it wasn't diagnosed
0:49:10 > 0:49:11until I got back.
0:49:11 > 0:49:14By which point I'd lost several stone in weight
0:49:14 > 0:49:19and was bleeding internally. It was pretty serious.
0:49:19 > 0:49:22It took me a while to kind of work out what I could eat
0:49:22 > 0:49:25- and what I couldn't.- Curry was not...
0:49:25 > 0:49:29Curry wasn't on the menu at all, no, I was limited to fish, rice,
0:49:29 > 0:49:34very plain foods, certain fruits, bananas I could eat.
0:49:34 > 0:49:38I had also a problem with my lips, they started crusting up,
0:49:38 > 0:49:41- which is another symptom of Crohn's disease.- Really?
0:49:41 > 0:49:44The immune system sort of attacks the whole digestive tract.
0:49:47 > 0:49:50Fed up with the side-effects from the drugs,
0:49:50 > 0:49:52Daniel turned to an unconventional treatment.
0:49:52 > 0:49:55Hookworm.
0:49:55 > 0:49:59These little bloodsucking parasitic worms live in the intestines.
0:50:01 > 0:50:05Daniel hoped they would suppress his gut's immune system
0:50:05 > 0:50:07for their own survival.
0:50:07 > 0:50:09And by doing so reduce his symptoms.
0:50:10 > 0:50:14Daniel ordered a tube of live hookworm on the internet
0:50:14 > 0:50:18and placed them on his skin, where they burrowed in,
0:50:18 > 0:50:20eventually finding their way into his gut.
0:50:22 > 0:50:24That was two years ago.
0:50:26 > 0:50:28How quickly did you notice changes?
0:50:28 > 0:50:31It was a gradual process over about a year.
0:50:31 > 0:50:34I realised I didn't have to use the cream on my lips.
0:50:34 > 0:50:39That was a real realisation that some kind of change had taken place.
0:50:39 > 0:50:42Were you spooked by the idea of having a parasite inside you?
0:50:42 > 0:50:45I wasn't, you know. I think human beings are designed
0:50:45 > 0:50:48to have some kind of parasites in them.
0:50:48 > 0:50:51It's part of modern life. We're so clean that we don't have any,
0:50:51 > 0:50:54but if you were living in a natural environment,
0:50:54 > 0:50:57in a situation that our bodies have evolved to live in,
0:50:57 > 0:50:59then you would have some kind of parasites going through you.
0:51:01 > 0:51:03I'm just going to try this.
0:51:03 > 0:51:08- Do you have any fear of food at the moment?- No, I'm really free.
0:51:08 > 0:51:09Thanks to the worms.
0:51:09 > 0:51:11THEY LAUGH
0:51:11 > 0:51:14I am basically free to eat as anyone else would.
0:51:17 > 0:51:20Daniel certainly makes a really compelling case for hookworm.
0:51:20 > 0:51:22I just wish there was some proper clinical trials
0:51:22 > 0:51:24that would support what he's doing,
0:51:24 > 0:51:28because there is always the risk that he happens to be exceptional,
0:51:28 > 0:51:32and also a risk that the hookworm might make your symptoms worse.
0:51:33 > 0:51:35But I'm very pleased it's worked for him.
0:51:37 > 0:51:40Helena Helmby, like most scientists,
0:51:40 > 0:51:43finds the idea of self infection very troubling.
0:51:44 > 0:51:49Scientists have worked for 100 years to eradicate these diseases
0:51:49 > 0:51:54and now we want to start reintroducing them into people again!
0:51:54 > 0:52:00These worms live in the gut and suck blood from your intestinal mucosa.
0:52:00 > 0:52:02They move around a lot as they do that,
0:52:02 > 0:52:04so they cause a lot of bleeding.
0:52:04 > 0:52:07If you have a high dose of hookworm in your gut,
0:52:07 > 0:52:09you will become anaemic quite quickly.
0:52:09 > 0:52:13Rather than infecting people with human parasites,
0:52:13 > 0:52:17we should looking into purifying these molecules
0:52:17 > 0:52:20that the worms produce and turn them into drugs,
0:52:20 > 0:52:22turn them into worm pills.
0:52:25 > 0:52:29The hope is that these studies will lead to new treatments
0:52:29 > 0:52:33for some of modern life's most debilitating conditions.
0:52:36 > 0:52:38My tapeworm, if it's there,
0:52:38 > 0:52:42has certainly been living in harmony with my body.
0:52:42 > 0:52:45Six weeks on and I still can't feel a thing.
0:52:48 > 0:52:52Of course, there is the possibility that the worm isn't there -
0:52:52 > 0:52:54that none of the three cysts survived.
0:53:00 > 0:53:03The best way to find out is using this.
0:53:03 > 0:53:06A miniature camera, which you swallow.
0:53:08 > 0:53:12OK. And I'm just going to place this on the front of your abdomen,
0:53:12 > 0:53:15and now I'm going to attach the data recorder,
0:53:15 > 0:53:19so that just plugs into the side here.
0:53:19 > 0:53:21So in your own time, if you want to swallow the capsule.
0:53:21 > 0:53:25- Never previously used. I'm assuming? - Never previously used!
0:53:25 > 0:53:26Down the hatch.
0:53:29 > 0:53:32- Right, I think that's done.- Gone.
0:53:32 > 0:53:36- And there's your tummy.- There's my stomach. That is weird, isn't it?
0:53:36 > 0:53:38There you go.
0:53:38 > 0:53:40Live from my stomach.
0:53:44 > 0:53:48Somewhere in the small intestine is the worm lurking, we hope.
0:53:49 > 0:53:54It's going to be quite strange to come face to face with it,
0:53:54 > 0:53:55so to speak.
0:53:58 > 0:54:02Before I can see the worm, I need to wait for the pill camera
0:54:02 > 0:54:06to pass out of my stomach and into my guts.
0:54:07 > 0:54:10So let's see where it's got to.
0:54:10 > 0:54:12It's still in the stomach.
0:54:12 > 0:54:14So it's hanging around a bit.
0:54:16 > 0:54:18It's not until the evening that the pill
0:54:18 > 0:54:21finally moves into my intestines.
0:54:22 > 0:54:26Ah! That... That is it.
0:54:26 > 0:54:28That is the tapeworm.
0:54:28 > 0:54:29You can see it.
0:54:29 > 0:54:31Oh, God, that is horrible.
0:54:31 > 0:54:35You can just see its tail and its segments, waving around.
0:54:35 > 0:54:38I don't know if I'm absolutely disgusted, or...
0:54:38 > 0:54:40I'm actually quite excited. Wow.
0:54:40 > 0:54:43Because it would have been hugely disappointing to have gone
0:54:43 > 0:54:47all the way to Kenya, come back and seen nothing.
0:54:47 > 0:54:48What's amazing is
0:54:48 > 0:54:51I've not experienced anything and yet you can see it there...
0:54:51 > 0:54:54That is so weird!
0:54:56 > 0:54:59Right. I wonder if I'm going to sleep tonight.
0:54:59 > 0:55:04I'm dying to go back and show it to my wife, see what she makes of it.
0:55:07 > 0:55:11This footage provides scientists with a rare chance to see
0:55:11 > 0:55:14a live worm in its natural habitat.
0:55:14 > 0:55:18So we sent the complete video to a team at Salford University.
0:55:18 > 0:55:22Just at the back there you can just see the thinnest
0:55:22 > 0:55:24part of the worm, this is the front end.
0:55:24 > 0:55:27The segments are getting more mature as you're moving down
0:55:27 > 0:55:29the length of the intestine - they're getting bigger,
0:55:29 > 0:55:31becoming sexually mature.
0:55:33 > 0:55:36Probably they're starting to get fertilisation taking place,
0:55:36 > 0:55:39so the worm is actually starting to reproduce at this stage.
0:55:39 > 0:55:44Can you tell at this point how big it is? I had no idea of the scale.
0:55:44 > 0:55:46By the looks of this it's certainly over a metre in length.
0:55:46 > 0:55:48So a metre?
0:55:48 > 0:55:51Yeah, certainly a couple of feet, probably three to four feet.
0:55:51 > 0:55:53Right. One of those.
0:55:56 > 0:55:59So this is the end of that worm.
0:55:59 > 0:56:01So it's worthwhile having a look further down the intestine
0:56:01 > 0:56:03- to see what's there.- Right.
0:56:09 > 0:56:11- Oh...- Ah! - MICHAEL LAUGHS
0:56:11 > 0:56:13That's another worm, yeah?
0:56:13 > 0:56:16You have more than one worm, definitely.
0:56:16 > 0:56:21Right. And so will it be competing with the other worm for nutrition?
0:56:21 > 0:56:24They are definitely in a different place,
0:56:24 > 0:56:27and it's quite possible that they are trying to avoid
0:56:27 > 0:56:30being in the same place so they're not competing with each other.
0:56:30 > 0:56:32There is a lot of nutrients in the intestine
0:56:32 > 0:56:35but the worms may well space themselves out deliberately
0:56:35 > 0:56:38so that they can get the most out of what you've been eating.
0:56:38 > 0:56:40I'm quietly thrilled.
0:56:40 > 0:56:43Slightly disgusted but quietly thrilled, I must admit.
0:56:44 > 0:56:46- I'll stop there.- OK.
0:56:46 > 0:56:50- And guess what, you've got three worms.- Whoa! Blimey.
0:56:50 > 0:56:55This is the very front end, the bit that attaches onto the intestine.
0:56:55 > 0:56:59There are actually four muscular suckers around that sort of head
0:56:59 > 0:57:01and those are what's latching onto
0:57:01 > 0:57:04- the wall of your intestine, keeping the worm in place.- Right.
0:57:04 > 0:57:06Wow. 100% hit rate.
0:57:06 > 0:57:10I have to say, a very successful experiment.
0:57:12 > 0:57:14With three worms inside me,
0:57:14 > 0:57:17I definitely don't want the segments crawling out.
0:57:17 > 0:57:19Time to treat my infection.
0:57:22 > 0:57:26Generally with worm infections a single dose of a medication does
0:57:26 > 0:57:29the job, so you're going to take those four tablets with some water.
0:57:29 > 0:57:30- All four?- All four of them.
0:57:30 > 0:57:33OK, down the hatch. Goodbye, worm.
0:57:41 > 0:57:43The pills kill the worms.
0:57:44 > 0:57:48Which are then absorbed by my gut.
0:57:49 > 0:57:53A suitably ironic end -
0:57:53 > 0:57:56parasites digested by their host.
0:57:57 > 0:57:59It has been absolutely eye-opening
0:57:59 > 0:58:02spending quality time with parasites,
0:58:02 > 0:58:05and discovering their ingenious strategies,
0:58:05 > 0:58:08from the relatively benign beef tapeworm
0:58:08 > 0:58:11to the frequently lethal malaria parasite.
0:58:13 > 0:58:16From the subtle manipulative skills of toxoplasma
0:58:16 > 0:58:19to the full-on assault of the leech.
0:58:19 > 0:58:20But above all
0:58:20 > 0:58:24I have learned enormous respect for these extraordinary creatures,
0:58:24 > 0:58:26and in future I will think twice
0:58:26 > 0:58:30before using the word "parasite" simply as a form of abuse.